Fuel source for a fuel cell

ABSTRACT

Fuel sources, fuel cells and methods of operating fuel cells are disclosed. In one aspect, the invention features a fuel source for a fuel cell, including a housing made substantially from a fuel-permeable material, and a fuel in the housing.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of and claims priority toU.S. Ser. No. 10/957,935, filed on Oct. 4, 2004, now, U.S. Pat. No.7,470,484, issued on Dec. 30, 2008, which is hereby incorporated byreference.

FIELD OF THE INVENTION

The invention relates to fuel sources, fuel cells and methods ofoperating the cells.

BACKGROUND

A fuel cell is a device capable of providing electrical energy from anelectrochemical reaction, typically between two or more reactants.Generally, a fuel cell includes two electrodes, called an anode and acathode, and a solid electrolyte disposed between the electrodes. Theanode contains an anode catalyst, and the cathode contains a cathodecatalyst. The electrolyte, such as a membrane electrolyte, is typicallyionically conducting but electronically non-conducting. The electrodesand solid electrolyte can be disposed between two gas diffusion layers(GDLs).

During operation of the fuel cell, the reactants are introduced to theappropriate electrodes. At the anode, the reactant(s) (the anodereactant(s)) interacts with the anode catalyst and forms reactionintermediates, such as ions and electrons. The ionic reactionintermediates can flow from the anode, through the electrolyte, and tothe cathode. The electrons, however, flow from the anode to the cathodethrough an external load electrically connecting the anode and thecathode. As electrons flow through the external load, electrical energyis provided. At the cathode, the cathode catalyst interacts with theother reactant(s) (the cathode reactant(s)), the intermediates formed atthe anode, and the electrons to complete the fuel cell reaction.

For example, in one type of fuel cell, sometimes called a directmethanol fuel cell (DMFC), the anode reactants include methanol andwater, and the cathode reactant includes oxygen (e.g., from air). At theanode, methanol is oxidized; and at the cathode, oxygen is reduced:CH₃OH+H₂O→CO₂+6H⁺+6e ⁻  (1)3/2O₂+6H⁺+6e ⁻→3H₂O  (2)CH₃OH+3/2O₂→CO₂+2H₂O  (3)As shown in Equation (1), oxidation of methanol produces carbon dioxide,protons, and electrons. The protons flow from the anode, through theelectrolyte, and to the cathode. The electrons flow from the anode tothe cathode through an external load, thereby providing electricalenergy. At the cathode, the protons and the electrons react with oxygento form water (Equation 2). Equation 3 shows the overall fuel cellreaction.

SUMMARY

The invention relates to fuel sources, fuel cells and methods ofoperating the cells.

In one aspect, the invention features a fuel source for a fuel cell,including a housing made substantially from a fuel-permeable material,and a fuel in the housing.

Embodiments may include one or more of the following features. Thefuel-permeable material has a fuel permeability of at least about 0.001g/hr·cm². The fuel-permeable material has a non-fuel permeability ofabout 10⁻³ g/hr·cm² for liquid non-fuel species or per atm of gaseousnon-fuel species or less. The fuel-permeable material includes aplurality of layers. The fuel-permeable material includes a first layerhaving a fuel permeability different than a fuel permeability of asecond layer. The fuel-permeable material includes a first layer havinga porosity different than a porosity of a second layer. Thefuel-permeable material is selected from the group consisting of afluorocarbon polymer, a modified polyethylene, a modified polypropylene,a polycarbonate, a polyimide, a polysulfone, a polysulfide, apolyurethane, a polyester, a polysilicone,poly(1-trimethylsilyl-1-propyne), and poly(4-methyl-2-pentene). Thefuel-permeable material defines at least about 60%, e.g., at least about80%, of an exterior surface area of the housing.

The housing can be capable of changing its volume. The housing caninclude a first layer of fuel-permeable material, and a second layer offuel-permeable material bonded to the first layer to define a cavity.The housing can include a portion having an initial surface areasubstantially corresponding to a surface area of an electrode of thefuel cell. The housing can be prismatic or cubic. The housing canconsist of the fuel-permeable material. The fuel source can furtherinclude a pressure-sensitive valve in fluid communication with aninterior of the housing. The housing can define the pressure-sensitivevalve. The housing can include a preferential folding portion.

The fuel can be in the form of a liquid or a gel. The fuel can includemethanol.

In another aspect, the invention features a fuel source for amethanol-powered fuel cell, including a collapsible housing made solelyfrom a fuel-permeable, non-fuel impermeable material, and a fuel in thehousing, the fuel comprising methanol.

Embodiments may include one or more of the following features. Thefuel-permeable, non-fuel impermeable material has a fuel permeability ofat least about 0.001 g/hr·cm². The fuel-permeable, non-fuel impermeablematerial has a non-fuel permeability of about 10⁻³ g/hr·cm² for liquidnon-fuel species or per atm of gaseous non-fuel species or less. Thefuel-permeable, non-fuel impermeable material is selected from the groupconsisting of a fluorocarbon polymer, a modified polyethylene, amodified polypropylene, a polycarbonate, a polyimide, a polysulfone, apolysulfide, a polyurethane, a polyester, a polysilicone,poly(1-trimethylsilyl-1-propyne), and poly(4-methyl-2-pentene). The fuelsource, wherein the housing comprises a portion having an initialsurface area substantially corresponding to a surface area of anelectrode of the fuel cell. The fuel source further includes apressure-sensitive valve in fluid communication with an interior of thehousing. The housing includes a preferential folding portion. Thefuel-permeable, non-fuel impermeable material includes a plurality oflayers.

The fuel can be in the form of a liquid or a gel.

In another aspect, the invention features a fuel cell system, includinga fuel cell comprising an anode, a cathode, and an electrolyte, a fuelsource configured to interface with the fuel cell, the fuel sourcecomprising a collapsible housing made substantially from a fuelpermeable, non-fuel impermeable material, and a fuel in the housing.

Embodiments may include one or more of the following features. Thefuel-permeable, non-fuel impermeable material has a fuel permeability ofat least about 0.001 g/hr·cm², and a non-fuel permeability of about 10⁻²g/hr·cm² for liquid non-fuel species or per atm of gaseous non-fuelspecies or less. The fuel-permeable, non-fuel impermeable material isselected from the group consisting of a fluorocarbon polymer, a modifiedpolyethylene, a modified polypropylene, a polycarbonate, a polyimide, apolysulfone, a polysulfide, a polyurethane, a polyester, a polysilicone,poly(1-trimethylsilyl-1-propyne), and poly(4-methyl-2-pentene). Thehousing includes a portion having an initial surface area substantiallycorresponding to a surface area of the anode of the fuel cell. Thehousing consists of the fuel-permeable, non-fuel impermeable material.The system further includes a pressure-sensitive valve in fluidcommunication with an interior of the housing. The fuel includesmethanol in the form of a liquid or a gel. The system further includes asupport configured to support the fuel source and to engage with thefuel cell.

In another aspect, the invention features a method of operating a fuelcell, including contacting an anode of the fuel cell with a fuel from ahousing made substantially from a fuel permeable, non-fuel impermeablematerial.

Embodiments may include one or more of the following features. Themethod further includes collapsing the housing. The method furtherincludes collapsing the housing along a preferential folding portion.The method further includes venting pressure from the housing.

Other aspects, features, and advantages will be apparent from thedrawing, description, and claims.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an embodiment of a fuel cell system.

FIG. 2 is a perspective illustration of an embodiment of a fuelcartridge including a fuel source and a support.

FIG. 3 is a perspective illustration of an embodiment of a fuel source.

FIG. 4 is a perspective illustration of an embodiment of a prismaticfuel source.

FIG. 5 is a perspective illustration of an embodiment of a cubic fuelsource.

FIG. 6 is a perspective illustration of an embodiment of a round fuelsource.

FIG. 7 is a perspective illustration of an embodiment of a fuel sourcehaving preferential folding lines.

FIG. 8 is a perspective illustration of an embodiment of a supportmember.

FIG. 9 is a perspective illustration of an embodiment of a supportmember.

FIG. 10 is a perspective illustration of an embodiment of a supportmember having a screen.

FIG. 11 is a perspective illustration of an embodiment of a supportmember having a restriction mechanism.

FIG. 12 is a discharge curve showing power density vs. time.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a fuel cell system 20, such as, a direct methanolfuel cell (DMFC) system, is shown. Fuel cell system 20 includes a fuelcell stack 22; a fuel system 24 in fluid communication with the fuelcell stack via a fuel inlet 26; a fuel outlet 28; a cathode reactant(e.g., air) inlet 30 in fluid communication with the fuel cell stack;and a cathode reactant outlet 32. For clarity, fuel cell stack 22 isshown having one fuel cell 34 (described below), but in otherembodiments, the fuel cell stack includes a plurality of fuel cells,e.g., arranged in series or in parallel. Briefly, fuel cell 34 includesan anode 36, a cathode 38, and an electrolyte 40 between the anode andthe cathode. Fuel cell 34 further includes two gas diffusion layers(GDL) 42 and 44, one disposed on each side of the electrolyte 40, anode36, and cathode 38 assembly.

Referring to FIG. 2, fuel system 24 is configured to deliver a vaporphase fuel to fuel cell stack 22. As shown, fuel system 24 includes asupport member 46 (e.g., a cartridge) and a fuel source 48 contained bythe support member. Referring also to FIG. 3, fuel source 48 includesone or more (as shown, two) substantially fuel permeable membranes 50that define an enclosed housing, and a fuel 52 (e.g., a fuel includingmethanol) in the housing. The membranes 50 are flexible and deformable,which allows fuel source 48 to deform during use as fuel 52 permeatesthrough the membranes and flows to fuel cell 34 for reaction. Morespecifically, fuel source 48 is capable of collapsing in volume as fuel52 is depleted during use, which can enhance the performance of fuelcell system 20.

For example, since fuel source 48 is capable of collapsing during use,adverse effects from a membrane expanding toward an anode can bereduced. In some embodiments of fuel cell systems, fuel system 24interfaces with a fuel cell stack or a fuel cell such that a membrane isspaced and adjacent to an anode. By allowing fuel source 48 to collapse,for example, such that the membrane does not expand toward or contactthe anode, the space between the anode and the membrane, which can besensitively associated with the performance of the fuel cell system, issubstantially maintained. In addition, maintaining the space between theanode and the membrane can reduce entrapment of condensed fluid (e.g.,water) between the anode and the membrane, which can also be detrimentalto fuel cell performance.

The ability of fuel source 48 to deform can also reduce the dependenceof the performance of the fuel source on the orientation of the fuelsource and/or fuel cell system 20. For example, referring to FIG. 2, asfuel 52 is consumed during use, fuel source 48 is capable ofself-folding or reducing in volume such that membrane 50 remains incontact with the fuel. As a result, the diffusion (e.g., permeation) offuel 52 through membrane 50 and to fuel cell 34 can remain relativelysteady because the interface between the fuel and the membrane canremain unchanged during fuel consumption. The fuel delivery rate is alsoenhanced by allowing the surface area of fuel 52 to contact fuelpermeable membrane(s) 50, compared to, for example, having the fuelspaced from the membrane(s) by a gas volume. In addition, theperformance of fuel source 48 can be independent of its orientationbecause, regardless of how the fuel source is oriented, fuel 52 remainsin contact with membrane 50, which can conform to the volume of thefuel. The ability of fuel source 48 to operate predictably anduniformly, independent of its orientation, provides the fuel source withthe versatility to be used in devices that may be moved or reorientedduring use.

In comparison, if membrane 50 of the fuel source 48 permits permeationof external non-fuel species and the fuel source does not collapseduring fuel consumption, then a gas volume (e.g., a gas bubble) can becreated in the fuel source as fuel 52 diffuses out of the fuel sourceand gas diffuses into the fuel source. The gas volume can affectperformance of the fuel source, depending on how the fuel source isoriented relative to the fuel cell. For example, still referring to FIG.2, if the fuel cell is below the fuel source (as shown in FIG. 2), thenfuel diffusion is from fuel 52, through membrane 50, and to the fuelcell. If the fuel cell is above the fuel source (as shown in FIG. 2),then fuel diffusion is from fuel 52, through a gas volume (assuming thegas volume is less dense than the fuel), through membrane 50, and to thefuel cell. If the fuel cell is to the side of the fuel source (as shownin FIG. 2), then, depending on the size of the gas volume, fueldiffusion from fuel 52 to the fuel cell may be affected by the gasvolume. In particular, if the gas volume contacts the portion ofmembrane 50 adjacent to the fuel cell, then fuel diffusion can beaffected by the gas volume since the gas volume is in the diffusion pathof the fuel to the fuel cell. Thus, the presence of a gas volume in afuel source can vary the performance of the fuel source as a function ofits orientation.

As indicated above, membrane 50 is substantially fuel permeable. Arelatively high fuel permeability can provide a high fuel delivery rate.Also, by forming fuel source 48 substantially of membrane(s) 50, fuel 52can contact a relatively high surface area of a fuel permeable material,which can further enhance fuel delivery rates. In some embodiments,membrane 50 has a fuel permeability of about 0.001 g/hr·cm² to about 2g/hr·cm² at 30° C. For example, the fuel permeability can be greaterthan or equal to about 0.001, 0.05, 0.10, 0.25, 0.50, 0.75, 1.00, 1.25,1.50, or 1.75 g/hr·cm² at 30° C.; and/or less than or equal to about 2,1.75, 1.50, 1.25, 1.00, 0.75, 0.50, 0.25, 0.10, or 0.05 g/hr·cm² at 30°C.

In addition, in some embodiments, membrane 50 is substantially non-fuelimpermeable. The non-fuel impermeability of membrane 50 restrictsnon-fuels from penetrating into fuel source 48, thereby diluting fuel 52and/or expanding the fuel source, both of which can reduce fuel cellperformance. Examples of non-fuels include components of any residualair (e.g., nitrogen and oxygen), anodically generated carbon dioxide,and water. Membrane 50 can have a non-fuel permeability of about 1×10⁻³g/hr·cm² for liquid non-fuels (or per atm pressure of gaseous non-fuelspecies) or less, e.g., less than about 1×10⁻⁴ g/hr·cm², less than about1×10⁻⁵ g/hr·cm², or less than about 1×10⁻⁶ g/hr·cm². A carbon dioxidepermeability of substantially less than 4×10⁻⁵ g-CO₂/hr·cm² atm-CO₂, forexample, can prevent formation of a bubble during discharge of a 20%efficient methanol fuel cell operating at 30 mW/cm² with equal membranearea and electrode area.

Membrane 50 can include (e.g., be formed wholly of) polymers. Examplesof polymers include fluorocarbon polymers, modified polyethylenes (e.g.,grafted polyethylenes), modified polypropylenes (e.g., grafatedpolypropylenes), polycarbonates, polyimides, polysulfones, polysulfides,polyurethane, polyesters, and polysilicones. Specifically,polydimethylsilicone (PDMS) and poly(trimethyl) silyl propynes (PTMSP)can provide high fuel delivery rates with relatively low non-fueluptake. Polyurethanes (e.g., one-mil thick from Deerfield Urethane, Inc.(Whately, Mass.)) can provide a high delivery rate, the ability to flexor collapse during fuel discharge, and a low gas permeability. Inaddition, the material can be amenable to thermal bonding, which can beused to fabricate fuel source 48. In some embodiments, the polymer(s)can be porous, for example, having a connected, continuous network ofpores to allow fuel transport.

In some embodiments, membrane 50 can include one or more additives toenhance the performance of the membrane. For example, a membraneincluding a polymer can include an additive that can plasticize thepolymer or provide other mechanical properties to enhance construction(e.g., fabrication and handling) of fuel source 48. Examples ofadditives include high molecular weight (non-volatile) glycols and otherplasticizer additives capable of influencing mechanical properties toenable flexibility and ease of manufacture. In other embodiments, havingno additives can enhance fuel diffusion (e.g., pervaporation) throughmembrane 50.

In some embodiments, membrane 50 can include a non-polymer. For example,membrane 50 can include a cellulose, such as paper. Other examplesinclude porous membranes that are fuel permeable, for example, includingmetal sheets, metal foils, such as sintered 316 stainless steelmembranes by the Mott Corporation, Farmington, Conn., or carbon layers.

The total thickness of membrane 50 can be a function of, for example,the particular material(s) of the membrane, the targeted fuelpermeability, the targeted non-fuel impermeability, and/or the targetedfuel cell performance. In some embodiments, a lower bound for thicknessis selected to provide a uniform and substantially defect-free (e.g.,substantially free of holes) membrane. An upper bound for thicknessexists because of volumetric considerations (e.g., to maximize theamount of fuel that can be stored in a given volume), and to provideflexibility so that the membrane collapse as the fuel is depleted.Membrane 50 can have a total thickness of about 100 nanometers to about200 micrometers.

Membrane 50 can include one homogeneous material or multiple layers ofone material or different materials. As an example, a composite membranestructure can include a relatively porous portion including PTMSP (e.g.,as the main layer in terms of thickness), and a relatively dense portionincluding PTMSP (e.g., a layer deposited on the main layer) to provideselectivity for methanol instead of water. A composite membranestructure is capable of imparting selectivity into membrane 50, whichcan be beneficial for reducing the impact that water permeation intofuel source 48 can have on methanol activity and thus fuel deliveryrate. Porous and dense polymers, such as PTMSP, are commerciallyavailable from Membrane Technology Research, Inc. (Menlo Park, Calif.).Examples of composite membrane structures are described, for example, in“Membrane Technology in the Chemical Industry” S. P. Nunes and K.-V.Peinemann, Eds., Wiley-VCH, 2001, pp. 149-151.

Fuel source 24 can be formed by bonding two or more membranes 50together to form a housing. For example, membranes 50 can be bondedtogether using an adhesive or by heat sealing (such as hot platepressing, melting around a mold, and/or laser bonding). The housing canbe formed in any shape configured for use with an intended fuel cellsystem. For example, referring to FIGS. 4-6, a fuel source 24 caninclude multiple membranes 50 bonded together to form a prismaticstructure (FIG. 4), a cubic structure (FIG. 5), or a round structure(FIG. 6). In some embodiments, the housing can include one unitarymembrane 50, e.g., formed by injection molding.

The housing can consist solely of membrane(s) 50, or the housing can bemade partially from the membrane(s). The latter option can be used toprevent fuel permeation in every external direction from fuel source 48.For example, one or more walls of fuel source 48 may be obstructed by aportion of an external cartridge housing or by an arrangement within thefuel cell system. The external obstruction may prevent locally permeatedfuel vapor from reaching the anode. Thus, to prevent fuel permeationfrom fuel source 48 to an inaccessible area, one or more sections of thefuel source can be fabricated from a material impermeable to fuel andnon-fuel species. Furthermore, if the material is flexible, thenon-permeable section(s) can provide or contribute to the ability offuel source 48 to collapse. In some embodiments, at least about 25%(e.g., at least about 30%, at least about 40%, at least about 50%, atleast about 60%, at least about 70%, at least about 80%, or at leastabout 90%) of the surface area of the housing is defined by membrane(s)50. The remaining surface area of the housing can be defined by thematerial chosen to be substantially impermeable to fuel and non-fuelspecies and perhaps flexible, such as polyethylene, polypropylene, andmodified copolymers of these or other materials.

In some embodiments, referring to FIG. 7, fuel source 48 includes one ormore preferential folding portions 54 (as shown, crease lines) formed inmembrane 50. Preferential folding portions 54 enhance volume shrinkingof fuel source 48 by allowing the fuel source to collapse in apredetermined manner, e.g., while maintaining a substantially fixed fuelpermeation active area during the life of the fuel source. Preferentialfolding portions 54 can be formed by removing portions of membrane 50,for example, by mechanically weakening, thinning (e.g., with an excimerlaser), and/or by scribing.

After the housing is formed, fuel 52, which is capable of providing fuelin gaseous form to fuel cell stack 22, can be placed in (e.g., injectedinto) the housing. Fuel 52, such as one including an alcohol (e.g.,methanol and/or ethanol) or a hydrocarbon, can be in the form of aliquid or a gel (e.g., non-gaseous form) having a vapor pressuresufficient to provide gaseous fuel to stack 22. A fuel gel is a viscousmaterial (e.g., from about 0.05 to about 200,000 centipoises) capable ofemitting a pure and high concentration of gas-phase fuel molecules. Theviscosity can be, for example, greater than or equal to about 10,000,25,000, 50,000, 100,000, or 150,000 centipoises; and/or less than orequal to about 200,000, 150,000, 100,000, 50,000, or 25,000 centipoises.An example of a fuel gel composition includes a fuel (e.g., methanol); adiluent (e.g., deionized water); a thickener (e.g., Carbopol EZ-3, anacidic, hydrophobically-modified, cross-linked polyacrylate powder); anda neutralizing agent (e.g., tri-isopropanolamine). Other fuel gels aredescribed in literature from Noveon that describe examples of the use ofCarbopol rheology modifiers (manufactured by BF Goodrich); andexemplified by cooking fuels (e.g., available from Sterno, andformulation examples listed by Noveon). A liquid fuel can include puremethanol, or a solution including methanol and water and/or gellingagent as non-fuel components. Fuel 52 can include additives, such asethanol, ethylene glycol, and/or formic acid, and fuel components. Insome embodiments, fuel 52 contains greater than about 90% by weight ofmethanol to maximize delivery rate and to minimize excess non-fuelvolume and mass, but greater than about 20% by weight of non-fuelcomponent(s) can be used to power a fuel cell.

Support member 46 can be of any shape configured to support fuel source48 and to interface with an intended fuel cell system. For example, asshown in FIG. 2, support member 46 can be replaceable cartridge that canengage with (e.g., snap fit to) to a fuel cell system. In otherembodiments, referring to FIG. 8, support member 46 can include one ormore channels 70 that provide fluid communication between where thesupport member is adjacent to (e.g., contacts) fuel source 48 and wherethe support member is adjacent to the fuel cell. Channels 70 provide adiffusion path between the fuel cell and portions of fuel source 48 thatwould otherwise be blocked by support member 46 (e.g., the sidewalls ofthe support member). Alternatively or additionally, referring to FIG. 9,to enhance fuel delivery from the sides of fuel source 48, portions ofsupport member 46 engaging the fuel source (e.g., the interior sidewalls) can be jagged, with the edges smoothed to prevent the fuel sourcefrom getting punctured.

Referring again to FIG. 1, an example of fuel cell 34 will now bedescribed. Fuel cell 34 includes electrolyte 40, anode 36 bonded on afirst side of the electrolyte, and cathode 38 bonded on a second side ofthe electrolyte. Electrolyte 40, anode 36, and cathode 38 are disposedbetween gas diffusion layers (GDLs) 42 and 44.

Electrolyte 40 should be capable of allowing ions to flow therethroughwhile providing a substantial resistance to the flow of electrons. Insome embodiments, electrolyte 40 is a solid polymer (e.g., a solidpolymer ion exchange membrane), such as a solid polymer proton exchangemembrane (e.g., a solid polymer containing sulfonic acid groups). Suchmembranes are commercially available from E.I. DuPont de Nemours Company(Wilmington, Del.) under the trademark NAFION. Alternatively,electrolyte 40 can also be prepared from the commercial productGORE-SELECT, available from W.L. Gore & Associates (Elkton, Md.).

Anode 36 can be formed of a material, such as a catalyst, capable ofinteracting with methanol and water to form carbon dioxide, protons andelectrons. Examples of such materials include, for example, platinum,platinum alloys (such as Pt—Ru, Pt—Mo, Pt—W, or Pt—Sn), platinumdispersed on carbon black. Anode 36 can further include an electrolyte,such as an ionomeric material, e.g., NAFION, that allows the anode toconduct protons. Alternatively, a suspension is applied to the surfacesof gas diffusion layers (described below) that face solid electrolyte40, and the suspension is then dried. The method of preparing anode 36may further include the use of pressure and temperature to achievebonding.

Cathode 38 can be formed of a material, such as a catalyst, capable ofinteracting with oxygen, electrons and protons to form water. Examplesof such materials include, for example, platinum, platinum alloys (suchas Pt—Co, Pt—Cr, or Pt—Fe) and noble metals dispersed on carbon black.Cathode 38 can further include an electrolyte, such as an ionomericmaterial, e.g., NAFION, that allows the cathode to conduct protons.Cathode 38 can be prepared as described above with respect to anode 36.

Gas diffusion layers (GDLS) 42 and 44 can be formed of a material thatis both gas and liquid permeable. Examples of GDLs are available fromvarious companies such as Etek in Natick, Mass., SGL in Valencia,Calif., and Zoltek in St. Louis, Mo. GDLs 42 and 44 can be electricallyconductive so that electrons can flow from anode 36 to an anode flowfield plate (not shown) and from a cathode flow field plate (not shown)to cathode 38.

Other embodiments of direct methanol fuel cells and fuel cell systems,including methods of use, are described, for example, in commonlyassigned U.S. Ser. No. 10/779,502, filed Feb. 13, 2004, and entitled“Fuel Cell”; “Fuel Cell Systems Explained”, J. Laraminie, A. Dicks,Wiley, New York, 2000; “Direct Methanol Fuel Cells: From a TwentiethCentury Electrochemist's Dream to a Twenty-first Century EmergingTechnology”, C. Lamy, J. Leger, S. Srinivasan, Modern Aspects ofElectrochemistry, No. 34, edited by J. Bockris et al., KluwerAcademic/Plenum Publishers, New York (2001) pp. 53-118; and “Developmentof a Miniature Fuel Cell for Portable Applications”, S. R. Narayanan, T.I. Valdez and F. Clara, in Direct Methanol Fuel Cells, S. R. Narayanan,S. Gottesfeld and T. Zawodzinski, Editors, Electrochemical SocietyProceedings, 2001-4 (2001) Pennington, N.J., all hereby incorporated byreference.

During operation of fuel cell system 20, fuel 52 from fuel system 24 isintroduced to anode 36, a cathode reactant (such as air) is introducedto cathode 38, and electrical energy is produced from the respectiveoxidation and reduction reactions as described above. In particular, asfuel 52 evaporates and permeates through membrane(s) 50, and diffuses toanode 36, fuel source 48 deforms to the decreasing volume of the fuel.This deformation can enhance the performance of fuel cell system 20 byallowing fuel 52 to be delivered in a uniform manner with reduced or noorientation dependence.

While certain embodiments have been described, the invention is not solimited.

For example, referring to FIG. 10, in some embodiments, support member46 can include a screen or a mesh 80 extending across the support memberand between fuel source 48 and the fuel cell. Screen 80 can restrictfuel source 48 from contacting, for example, the anode of the fuel cell.

Alternatively or additionally, referring to FIG. 11, fuel system 24 caninclude one or more restrictive mechanisms that partially or completelyreduce (e.g., isolate) fuel flow from the fuel system to the fuel cellwhen the fuel cell system is not in use. The restrictive mechanism canrestrict fuel 52 from unwanted loss, and from accumulating in an anodechamber and migrating across electrolyte 40 to cause parasitic losses(thereby reducing runtime) and mixed potentials at cathode 38 (therebyreducing output power). An example of a restrictive mechanism is apressure-sensitive valve 90 (as shown, a slit valve made from a polymermembrane) extending across the support member and between fuel source 48and the fuel cell. The pressure-sensitive valve can automatically openwhen there is a pressure differential (e.g., from accumulated fuelvapor), and close when the pressure differential is eliminated (e.g.,when the fuel is vented). Pressure-sensitive valves are described, forexample, in U.S. Ser. No. 10/236,126, filed Sep. 6, 2002.

In some embodiments, fuel source 48 can be used without a supportmember.

The following examples are illustrative and not intended to be limiting.

Example 1

This example illustrates fabrication of a liquid methanol fuel sourceand discharge within a vapor feed DMFC.

A fuel source or pocket was constructed using about 2.25 in² (14.5 cm²)of 1-mil thick polyurethane from Deerfield Urethane, Inc of Whately,Mass. The sides of the square-shaped pocket were heat bonded on threesides using an Impulse Sealer (Type AIE-200 from American InternationalElectric) filled with 2.26 g of neat methanol, and then closed bysealing the final edge.

Next, the fuel pocket was placed in a single cell vapor feed DMFC,specifically within a closed cavity adjacent to the anode to allow thefuel vapors to reach the anode. The membrane electrode assembly of thecell included a Nafion® polyelectrolyte membrane (commercially availablefrom E.I. DuPont de Nemours Company, Wilmington, Del.), a Pt/Ru anode,and a Pt cathode of 5 cm² electrode active area. The cell was subjectedto 0.3V discharge with periodic sweeps to 0.18 V and then to opencircuit. The fuel cell was run for 20 h in a “face up” orientation, thatis, with the fuel pocket below the cell. Controlled ambient conditionswere used in the run: 26° C. and 30% humidity. After completing theexperiment, almost no liquid (<0.5 g) remained in the pocket. The pocketvolume reduced substantially, conforming to the small amount of liquidremaining. This observation indicated the desirable rejection of gasinto the fuel pocket. FIG. 12 shows the discharge curve (time vs. powerdensity) over the 20 hr potentiostatic operation.

Example 2

This example illustrates fabrication of a fuel source or pocket having agelled methanol fuel.

Four distinct fuel pockets, each roughly 3.0×1.5 inches (4.5 in² or 290cm²), were heat bonded using an Impulse Sealer (Type AIE-200 fromAmerican International Electric) and filled with an unmeasured amount ofpre-made gelled methanol. A 3.0×3.0 inch sheet of 1-mil thickpolyurethane film from Deerfield Urethane, Inc. of Whately, Mass. wasfolded in half to form the 3×1.5 in rectangle and sealed on two edges tobegin fabrication of the pocket. The pocket was sealed at the ends withtwo 1.5×1.5 inch panels. An unmeasured amount of pre-made gelledmethanol fuel was placed through the open edge filling the pocket tocapacity. (The gelled fuel consisted of approximately 1.66 wt % CarbopolEZ-3, 0.04% NaOH and 98.30 wt. % methanol.) The final edge of the pocketwas sealed with care to minimize trapping of air. The height of thefilled pockets were approximately 0.3 to 0.4 in when placed on a flatsurface, spread, and leveled.

Example 3

This example illustrates fabrication of a fuel source includingsubstantially of non-fuel permeable material.

A semi-flexible cubic structure was heat sealed using an Impulse Sealer(Type AIE-200 from American International Electric) from two sheets of5-mil thick polyethylene, a flexible, collapsible and substantiallymethanol impermeable material. Each flattened face of the resultant cubehad dimensions of 1¾×1¾ inch.

In other embodiments, the cube can include a face cutaway for fuel vaporescape to a fuel cell anode. The cutaway section can be left open, orfor orientation independence, be covered with fuel permeable membrane byheat or mechanically sealing the edges of the fuel permeable and fuelimpermeable sections.

Example 4

This example illustrates the mechanical robustness and seal integrity ofa liquid methanol fuel pocket.

Two distinct fuel pockets were constructed using an Impulse Sealer (TypeAIE-200 from American International Electric) and filled with anunmeasured amount of liquid methanol. Two roughly 4×3.0 inch sheets of1-mil thick polyurethane film from Deerfield Urethane, Inc. of Whately,Mass. were each folded in half to form a 2×1.5 in rectangles and eachsealed on two edges to begin fabrication of the pocket. The pocket wassealed at the ends with two 1.5×1.5 inch panels. An unmeasured amount ofliquid methanol fuel was placed through the open edge filling eachpocket to capacity. Held upright, the final edge of the pockets werepinched at the liquid level to minimize trapping of air and sealed.Lying on a flat surface, each liquid pocket self-leveled to a height ofroughly 0.5 inches and an area of roughly 2×3 inches.

To verify seal integrity qualitatively, each pocket was rested upon drytissue paper for duration on the order of seconds in several differentorientations. Upon lift of each pocket, no trace of liquid was observed,indicating negligible leakage of methanol liquid from the pocket. Overtime, the pocket reduced in size due to the permeation of methanol vaporfrom the membrane surface.

To verify mechanical robustness qualitatively, one of the pockets wasdropped from a vertical distance of greater than 3 feet and re-testedfor leaks by resting upon dry tissue. Again, upon lift of the pocket, notrace of liquid was observed indicating non-leakage of liquid methanol.

All references, such as patent applications, publications, and patents,referred to herein are incorporated by reference in their entirety.

Other embodiments are in the claims.

1. A fuel source configured to provide fuel in vapor form to a fuelcell, comprising: a collapsible housing and a fuel selected from thegroup consisting of methanol, ethanol, and hydrocarbons within thehousing, wherein the housing has an exterior surface and includes amembrane that is permeable to the fuel in vapor form and defines atleast 25% of the exterior surface area, and wherein as the fuel in vaporform passes through the membrane the housing collapses.
 2. The fuelsource of claim 1, wherein the membrane has a permeability to the fuelin vapor form of at least about 0.001 g/hr·cm².
 3. The fuel source ofclaim 1, wherein the membrane comprises a plurality of layers.
 4. Thefuel source of claim 3, wherein the membrane comprises a first layerhaving a fuel permeability different than a fuel permeability of asecond layer.
 5. The fuel source of claim 3, wherein the membranecomprises a first layer having a porosity different than a porosity of asecond layer.
 6. The fuel source of claim 1, wherein the membranecomprises a material selected from the group consisting of afluorocarbon polymer, a modified polyethylene, a modified polypropylene,a polycarbonate, a polyimide, a polysulfone, a polysulfide, apolyurethane, a polyester, a polysilicone,poly(1-trimethylsilyl-1-propyne), and poly(4-methyl-2-pentene).
 7. Thefuel source of claim 1, wherein the housing comprises an additionalmembrane, and the membrane and the additional membrane are bonded todefine a cavity.
 8. The fuel source of claim 1, wherein the housing isprismatic or cubic.
 9. The fuel source of claim 1, further comprising apressure-sensitive valve in fluid communication with an interior of thehousing.
 10. The fuel source of claim 9, wherein the housing defines thepressure-sensitive valve.
 11. The fuel source of claim 1, wherein thefuel is in the form of a liquid.
 12. The fuel source of claim 1, whereinthe fuel is in the form of a gel.
 13. The fuel source of claim 1,wherein the membrane defines at least about 60% of the exterior surfacearea of the housing.
 14. The fuel source of claim 1, wherein themembrane defines at least about 80% of the exterior surface area of thehousing.
 15. The fuel source of claim 1, wherein the housing comprises apreferential folding portion.
 16. The fuel source of claim 1, whereinthe membrane has a thickness of between 100 nanometers and 200micrometers.
 17. The fuel source of claim 1, wherein the membrane isimpermeable to water and carbon dioxide.
 18. The fuel source of claim 1,wherein the membrane is impermeable to nitrogen and oxygen.
 19. The fuelsource of claim 1, wherein the fuel is methanol.